The present invention relates to treads for radial carcass tires intended for heavy vehicles, more particularly buses.
Due to the very draconian conditions caused by the use of studded tires, it has appeared necessary, in order to avoid damage to the roads or even a total prohibition against the use of studded tires, to provide tires intended for heavy vehicles, and in particular buses, which are excellent in all weather, in all positions of the vehicle and, in particular, on the driving axles. It is well known that the tread of all-weather radial tires for heavy vehicles is generally formed of individual blocks arranged in several circumferential rows so as to provide excellent traction. However, such a tread rapidly leads to abnormal wear which may result in premature retirement of the tires due to the noise produced upon travel.
In order to overcome this drawback, U.S. Pat. No. 4,676,290 has proposed a tread formed of individual blocks with recesses on each of their circumferential edges, which are separated in circumferential direction by narrow grooves of generally transverse orientation which are of reduced depth as compared with those of the circumferential grooves and form platforms between the blocks in the circumferential direction. These identical platforms are present in all the rows of blocks with the exception of the side blocks of the tread. Such a solution has the drawback of offering only small driving traction on snow-covered ground.
It is furthermore known from British Patent 2 120 183 to improve the adherence of heavy vehicles on wet ground when they are under low load by using a tread formed of several rows of individual blocks present in the median zone and separated from each other in the circumferential direction by platforms arranged in narrow transverse grooves, the platforms in the vicinity of the median axis of the tread having, at least in the part close to the median axis, the smallest height and a greater height in the part directed towards the edges of the tread. This solution is no more satisfactory than the preceding one from the standpoint of adherence to snow-covered ground.
European Patent Application 355 636 describes a heavy- vehicle tire tread which retains its initial braking properties even after half-wear, which has, in the median zone of the said tread, fine incisions of a depth greater than those present in the side zones and a density less than the density of incisions present in the side zones.